/Engine/Source/Runtime/MediaAssets/Public/MediaPlayerProxyInterface.h
This should be called every frame until it returns true. It might take a few frames to discover what the aspect ratio is. Also some proxies might not support aspect ratios, so they will also return true so you do not call this endlessly.
True if you no longer need to call this.
bool ProxySetAspectRatio ( [UMediaPlayer](API\Runtime\MediaAssets\UMediaPlayer) * InMediaPlayer )